Key Engineering Materials Vol. 716

Abstract: For a metal forming process, the uniaxial tensile properties of a material are the most fundamental and important properties to investigate. Solution heat treatment, forming and in-die quenching (HFQ^®) is a patented process to form complex shape panel components using aluminium alloys at high efficiency and low cost. A Gleeble materials thermo-mechanical simulator was used to conduct uniaxial tensile testing of AA6082 under HFQ^® conditions. A set of grips were specially designed to reduce the heat loss of specimen during testing in a Gleeble and allow the strain measurement by using digital image correlation (DIC) system. A large dog-bone specimen with parallel length of 80mm was designed to minimise the temperature gradient along the gauge section. Temperature gradient was measured and uniaxial tensile tests were conducted at the range of deformation temperature of 350-535 °C and the range of strain rate of 0.1-4 /s. The uniaxial tensile properties of AA6082 at different temperatures and strain rates under HFQ^® conditions were summarised and the viscoplastic response of the material was discussed.

Abstract: The aim of this study is to reveal the influence of ultrasonic treatment in the molten state on microstructural evolution of the magnesium alloy AZ31 with and without the addition of SiC particles. Therefore, a heatable wedge-shaped mold for holding the magnesium melt on the required temperature was constructed with the possibility to insert an ultrasonic horn into the melt. Previously ingots of AZ31 were molten in an electrical furnace under a protective gas atmosphere. SiC particles with an average diameter of 2 μm were added to the melt. Mechanical stirring was conducted to ensure homogeneous distribution of the particles. The molten Mg was subjected to ultrasonic sound with constant frequency of 20,000 Hz and amplitude of 12.4 μm. The time of ultrasonic treatment was defined on 60 seconds. The material solidified quickly due to the cooling with water nozzles after withdrawing of the ultrasonic horn. AZ31 without SiC particles was treated under the same conditions. The resulting microstructure was observed by optical and scanning electron microscopy. Depending on resulting intensity and the effective area a refinement in grain size and also more homogeneous distribution of precipitations in the material is achieved. The resulting microstructures of AZ31 with and without SiC addition were compared and discussed.

Abstract: The present study details the results of finite element analysis (FEA) based predictions for microstructure evolution in ATI 718Plus^® alloy during the hot deformation process. A detailed description of models for static grain growth and recrystallisation is provided. The simulated average grain size is compared with those experimentally measured in aerofoil parts after forging trials. The proposed modified JMAK model has proved to be valid in the main body of the forging. The results predicted for the surface are less accurate. The recrystallised grain size on the surface is smaller than in the centre of the part which corresponds to the experimental results and reflects the main trend.

Abstract: The present investigation aims at studying the flow behaviour of magnesium alloys under different conditions in terms of temperature, deformation velocities and deformation. The modelling approach was based on a proposed equation to model the shape of each flow curve through different variables. The modelled flow curves were subsequently compared with those obtained with experiments. The models were validated on flow curves not used in the building stage. It was observed that, for low temperature values, high deformation velocities and deformations the final part of the flow curve has to be adapted in order to be adopted for the description of material in the numerical simulation. In other words it needs to be extrapolated. Also for the high temperature, the flow softening has to be limited in order to allow the extrapolation queue required for elevated deformations. The deformation value at which the extrapolation can start can be predicted with an other proposed equation detailed in the paper.

Abstract: We present calculations of austenite to bainite phase transformation start for different cooling paths and for different steel compositions and a method to estimate the cooling water required to cool a steel strip to desired temperatures during water cooling line after industrial hot rolling. We also quantitatively compare how different alloying elements affect the phase transformation activation energy and the time required for the transformation to start and proceed to the extent that it can be detected with dilatometer. This analysis can be used for aid when designing suitable cooling paths for hot rolled steel products. The calculations of the activation energy can be used as input in more detailed microstructure models.

Abstract: In multiple stages forging, an appropriate pre-form has reduced production cost, loss of material, and forging load, and it has also improved die life. However, it is difficult for beginner to make the process design of the pre-form because they have poor technical information. For example, experts have experimental knowledge how to form an oval cylinder by upsetting of a right cylinder with a mild-wedged die. This oval upsetting needs no container with oval hole. This paper investigates the oval upsetting with the relationship between the nose angle of the mild-wedged die and the shape of the product for assistance of the process design in forging. As the results, when the mild-wedged die having an appropriate angle and nose radius was applied to the upsetting, the oval cross-sectional ratio of the major axis to the minor axis achieved 1.45.

Abstract: In industry the increasing variety of products leads to shorter product life cycles. For parts made by forging processes this trend results in very high prizes, as the tool costs have to be assimilated with only few parts. To reduce the tool costs new, flexible processes have to be investigated and established in tool manufacturing. Laser based additive manufacturing is noted for its high flexibility and especially laser metal deposition (LMD) gets in the focus of the research as it allows adding material on free formed surfaces. Therefore it is already used for coating and repairing of forming tools. New investigations are made to qualify this process for the production of forging tools. The aim is to generate active elements onto a geometrical simple base unit. Within first investigations the manufacturing of high carbon hot work tool steel 1.2343 was analysed. The measured mechanical properties were similar to those of conventional manufactured material.The focus of this paper is the connection strength of the additively built structures to the substrate. Therefore cylindrical specimens for tensile tests are manufactured with the linkage zone in the parallel area, in which the highest tension will be achieved. To assess the strength of the connection a comparison with conventional manufactured steel will be made. Furthermore specimens produced with various settings will be tested to analyse the influence of the LMD process. Additionally post heat treated samples will be analysed to recognize the effect of the hardening on the strength of the specimens.

Abstract: The formability and geometrical accuracy in incremental sheet forming can be increased using a force-controlled support tool. The main problems in using such a kinematic support tool is the positioning of forming and support tools, while maintaining force magnitude and alignment. A new tool for this was developed systematically using a morphological box. It uses a spring controlled rotation of the tool tip to maintain the force. Since the rotation axis is not in line with the tool tip axis and the tool tip can freely rotate around its axis, roll friction conditions can be achieved. The center of gravity of the rotating part of the tool is placed in the rotating axis and the force is therefore independent from the alignment of the tool in space. It has a mechanical stop with locking option in the vertical position and is therefore also fully usable as a forming tool. While the prototype is manually controlled, concepts for a fully automated version have been designed, as well. First tests are in line with results described in literature, showing that direction and magnitude of force have an influence on the formability.

Abstract: Hot stamping of aluminium alloys is a tailored forming process, with the assigned processing windows determining the quality of each hot stamped component in terms of its post-form strength. In this work, a processing window calculator, ‘Tailor’, was developed in order to define the optimal processing parameters that should be used in a production line to successfully produce a component with the desired post-form strength using hot stamping. ‘Tailor’ was developed using the results of forming tests, air-cooling tests and multi-stage artificial ageing tests, which provided guidance on the values for the die closing force, transfer time and artificial ageing time to be used in the hot stamping process. The effectiveness of ‘Tailor’ was demonstrated in two case studies.

Abstract: The roll levelling and coil straightening facilities are becoming one of the most important process steps when forming Ultra High Strength Steels. The correct levelling and straightening of these materials are the main responsible for the stabilization of the residual stresses through the thickness and post forming springback of sheet metal formed components.Due to the arrangement of the rolls in roll levellers, micro-sliding occurs between the rolls and the sheet and the first rolls are subjected to high torques and high forces since they are responsible for the plastification of the material to a high extent. In order to reduce these drawbacks and optimize the energy consumption avoiding energy loses due to friction, the use of servo technology in levelling processes is studied in this paper.A wide range of materials are levelled using the conventional and the new servo controlled strategy that uses two servo motors and the process improvements are quantified.